Fluid back pressure damper

ABSTRACT

A fluid back pressure damper having at least one counterbalanced pivoted blade which is swung open by the flow of a gaseous fluid against the blade and, under normal conditions, which is swung closed by the mass of a counterweight when the gaseous flow ceases. The blade is mounted on a transverse shaft along a line spaced from its median line and is fabricated from flexible sheet material. The counterweight is connected to the blade shaft by means providing for relative angular movement between the blade and the counterweight. In normal closed position the blade closes against a stop or, when there is a plurality of blades in the damper, each blade closes against the adjacent blade and/or its shaft which thus functions as a stop. If and when a sudden back pressure occurs, the abrupt gaseous flow impinges on the blade. Because of the relative angular movement between the blade and the counterweight, the blade starts to move toward the closed position almost unhampered by the inertia of the counter-weight. The blade swings closed, sharply striking its stop and flexes upon impact. Because the blade and the counterweight can move angularly relative to each other, the counterweight moves beyond its normal closed position in order to absorb at least a part of the kinetic energy of the counterweight. A shock absorbing means may be positioned in line to be impacted by the counterweight after passing its normal closed position in order to absorb any remaining kinetic energy of the counterweight.

yet.'-

States tent [54] FLUID BACK PRESSURE DAMPER Inventors: Raymond L; Alley, Hartman, Jr.,

Toledo, all of Ohio The American Warming Inc., Toledo, Ohio Fued; sept. 21, 1970 App1. No.. 73,727

Assignee:

U.S. Cl. Int. (3L-...3.... Field of Search References Cited UNITED STATES PATENTS Primary Examiner-Edward J. Michael Attorney-Owen & Owen Steiner..

Toledo; George F. Maumee; John D. Ewald,

& Ventilating,

.4.98/110, 98/119 F24f 13/14 ..98/l l0, l 19, l2l; 236/45 ..98/1 1o x ....98/119 ....236/45 N236/45 ...9s/119 x 236/45 5 7] ABSTRACT A tluid back pressure damper having at least one counterbalanced pivoted blade which is swung open by the flow of a gaseous fluid against the blade and, under normal conditions, which is swung closed by the mass of a counterweight when the gaseous flow ceases. The blade is mounted on a transverse shaft along a line spaced from its median line and is fabricated from flexible sheet material. The counterweight is connected to the blade shaft by means providing for relative angular movement between the blade and the counterweight. ln normal closed position the blade closes against a stop or, when there is a plurality of blades in the damper, each blade closes against the adjacent blade and/or its shaft which thus functions as a stop. lf and when a sudden back pressure occurs, the abrupt gaseous flow impinges on the blade. Because of the relative angular movement between the blade and the counterweight, the blade starts to move toward the closed position almost unhampered by the inertia of the counter-weight. The blade swings closed, sharply striking its stop and lexes upon impact. Because the blade and the counterweight can move angularly relative to each other, the counterweight moves beyond its normal closed position in order to absorb at least a part of the kinetic energy of the counterweight. A shock absorbing means may be positioned in line to be impacted by the counterweight after passing its normal closed position in order to absorb any remaining kinetic energy ofthe counterweight.

12 Claims, 11 Drawing Figures FLUIDBACK PRESSURE DAMPER EACKGROIJND OF TI-IE INVENTION In many commercial, industrial and scientic establishments it is necessary to Asupply large volumesof air or other gaseous fluids to enclosures containing machinery or apparatus requiring special atmospheres. The gaseous fluids may be provided for the purpose of controlling temperature, relative humidity, pressure, or to maintain an inert atmosphere as to prevent explosions, or the like. Such enclosures may also be supplied with other fluids or liquids, often under extremely high pressures and temperatures, such as super-heated steam in order to actuate or controlequipment or apparatus, or to flow from the equipment or apparatus to additional devices actuated thereby. It is necessary not only to have ducts for supplying the gaseous fluid to the enclosure but means must be provided for rapidly closing such ducts in the event of the creation of an excessively highpressure within the enclosure, as for example, by the rupture of a high pressure steam line or the like.

It is therefore the principal object of the instant invention to provide a damper for a gaseous fluid supply duct which will be held open by the normal flow of gases through the duct in an enclosure, which will close automatically in the event that the supply of gases is deliberately stopped and which will close very rapidly in the event of the occurrence of an excessively high pressure within the enclosure and, upon closing under such conditions, will withstand a substantial back pressure.

It is a further object of the instant invention to provide a damper fora gaseous fluid supplyduct which will close extremely rapidly, say in from 0.05 to 0.10 seconds, and which will absorb the high kinetic energy created by its rapid movement to closed position without damage to the damper itself, the ductwork or to the mounting of the damper.

DESCRIPTIONS OF THE DRAWINGS FIG. 1 is a fragmentary view, partly in section and partly in elevation, and with parts broken away, of a damper embodying the invention; v

FIG. 2 is a plan view of a damper embodying the invention, taken from the position indicated by the line 2--2 of FIG. 1 and shown on a slightly enlarged scale;

FIG. 3 is a fragmentary, vertical, sectional .view taken along the line 3--3 of FIG. 2, and shown on a still further enlarged scale;

FIG. 4 is a fragmentary view in elevation, with parts broken away, taken from the position indicated by the line 4-4 of FIG. 3 shown on a still further enlarged scale and illustrating the operation of a damper according to the invention under extreme back pressure conditions;

FIG. 5 is a fragmentary detailed view in vertical section taken along the line S-S of FIG. 4;

FIG. 6 is a fragmentary, somewhat diagrammatic view similar to FIG. 4, but showing the damper in normally closed position;

FIG. 7 is a fragmentary view partly in section of a back pressure damper embodying the invention as fitted into a circular duct and having only one damper blade.

FIG. 8 is a horizontal sectional view taken along the line 8- 8 of FIG. 7;

FIG. 9 is a fragmentary, vertical sectional view taken along the line 9-9 of FIG. 8 and shown on an enlarged scale;

FIG. 10 is a view similar to FIG. 8, but of another damper embodying the invention as fitted into a square duct, and having only one damper blade; and

FIG. 11 is a fragmentary detailed view on an enlarged scale taken along the line 11-11 of FIG. 10 but showing the damper blade in a position different from that shown in FIG. l0.

DESCRIPTION OF THE PREFERRED EMBODIMENT A back pressure damper embodying the invention is generally indicated by the reference number l0 and, in FIG. l,

is shown in position between a source duct 11 and a delivery duct 12. Suitably conditioned air or other gaseous fluid is supplied to the source duct 11 from a fan, air conditioning apparatus, compressor, gaseous supply or the like, and is fed through the source duct 11 at suicient pressure to flow through the delivery duct 12 to the enclosure to which it is supplied. A damper 10 embodying the invention, as is the case with normal air flow dampers, is so designed and constructed as to open in response to the flow of gases, as indicated by the arrows in FIG. 1, from the source duct 11 to the delivery duct 12 and to remain open as long as the gas supply is maintained.

The damper 10 has an open rectangular frame 13 comprising four channels 14 to the flanges of which there are secured end flanges 15 of the source and delivery ducts 1 1 and l2. The four channels 14 of the damper 10 thus define a portion of the main gas passageway leading from the source of gas to the enclosure to which the gases are delivered. In the embodiment of the invention illustrated FIGS. 1 to 6 of the drawings, a box girder (see also FIG. 3) extends across the frame 13 dividing the frame 13 into two parallel rectangular openings 17 and 18 through which the gases actually flow.

Each of the openings 17 and 18 is controlled by a plurality of damper blades 19 and each of the blades 19 is mounted upon a shaft 20, each of which comprises a square tube 21. The blade shafts 20 are mounted in spaced parallel relationship extending transversely across the respective opening 17 or 18. At their inner ends, the shafts 21 also comprise tenons 22 having square bodies fitting interiorly of the tubes 21 and cylindrical ends which are journaled in bearings 23 mounted in vertical webs 24 of the box girder 16. At their outer ends each of the shafts 20 comprises a similar tenon 25 which similarly has a square body and an elongated rod 26 by which the respective shaft 20 is journaled in a similar bearing 27 mounted on the other side of the respective one of the channels 14. The tubes 21 are secured to the square bodies of their respective tenons 22 and 25 by pairs of bolts 28 and the bolts 28 also mount the damper blades 19 on the respective ones of the shafts 20.

`Each of the damper blades `19 preferably is fabricated from spring steel and has a right angle flange 29 (FIG. 4) lying along its respective tube 21 and held thereon by the bolts 28. Each of the blades 19 is long enough to extend substantially across the respective one of the openings 17 and 18 and wide enough to extend from its own shaft 20 into engagement with the next adjacent one of the shafts 20 when the damper is closed as illustrated in FIG. 4 or 6. The lateral edge 30 of each damper blade 19, remote from its shaft 20, engages the adjacent one of the damper blade shafts 20 or a transverse angle stop 31 (FIG. 4) mounted on the inner side of a frame channel 14 at one end ofthe opening 17 or 18 (at the right in FIGS. 2 and 4).

At the outboard end of each of tenon rods 26 there is pinned or otherwise secured a link arm 32 and all of the link arms 32 are pivotally connected by shoulder bolts 33 (FIG. 5) to a tie bar 34. As can best be seen in FIG. 1, the tie bars 34 limit the open position of the damper blades 19 (when the blades are swung open by the flow of gases through the damper 20) by engaging stop brackets 35 which extend outwardly from the damper frame 13 on each side in line to be engaged by the ends of the tie bars 34 for the respective set of damper blades 19 in the two openings 17 and 18.

A counterweight arm 36 is welded or otherwise rigidly mounted on each of the tenon rods 26 extending generally radially oppositely from the respective one of the damper blades 19. Near the outer end of each of the counterweight arms 36 there is cut a transverse slot 37 and circular counterweights 38 are carried by a tie rod 39 which extends through the slot 37.

Energy absorbing blocks 40, fabricated from a tough resilient material such as butyl rubber, are mounted on horizontal brackets 41 which extend outwardly from the respective one of the outboard frame channels 14, and which are rigidifled by gusset plates 42. The blocks 40 are in line to be impacted by the counterweights 38 under unusual conditions resulting from the occurrence of a sudden back pressure within the enclosure to which the gases normally are fed.

Under normal operating conditions, when the gases are being fed from the source duct 11 through the damper 10 to the delivery duct 12, the flow of the gases swings the damper blades 19 to open position elevating the counterweight arms 36 upwardly into the position illustrated in FIG. 1. In this position the counter-weights 38 on their tie rods 39 drop to the bottoms of the slots 37 and the position of the damper blades 19 is limited by the engagement of the tie bars 34 with their stop brackets 35. The mass of the counter-weights 38 is so selected that the 'tendency for the counterweights 38 to swing the damper blades 19 to the closed position illustrated in FIG. 6 is overcome by the force of the gases being fed through the damper 20 and into engagement with the upper or inner surfaces of the damper blades 19.

When the source of the gases, for example the fan, air conditioning apparatus, or the like, is turned off so that gases no longer are fed through the source duct 11, the counterweights 38 act to swing their respective counterweight arms 36, damper shafts 20 and damper blades 19 to the closed position as illustrated in FIG. 6. This position is limited by the engagement of the edges 30 of each ofthe damper blades 19 with the next adjacent damper blade shaft 20 or the stop 31 as the case might be. In this normally closed position, as can be seen in FIG. 6, the counterweights 38 rest in the bottoms of their respective slots 37 and do not engage the surfaces of the energy absorbing blocks 40.

lf the damper 20 is open, as illustrated in FIGS. 1 and 2, and a sudden back pressure is created in the enclosure to which the gases are being delivered, as for example by the rupture of a high pressure steam line or the like, the high pressure thus created in the enclosure acts as a back pressure coming up the delivery duct 12. These gases impinge against the damper blades 19 and swing the damper blades 19 to closed position (counterclockwise in FIGS. 1 and 4). As the blades 19 start to swing toward closed position, they are accelerated at a high rate of speed particularly as they extend further across the path of movement of the back pressure gases. Because of the inertia of the counterweights 38, the counter-weight arms 36, blade shafts 20 and blades 19 rotate angularly relative to the counterweights 38 and the counterweight tie rods 39 rise to the tops of the respective slots 37.

The energy resulting from the extremely high speed closing of the damper blades 19 causes their edges 30 to impact against the adjacent blade shafts 20 or the stop angle 3l with considerable force and the energy thus delivered causes each of the damper blades 19 to flex upwardly as is illustrated in FIG. 4. As a result the damper blade shafts 20 rotate in a counterclockwise direction (FIG. 4) a number of degrees beyond the position assumed in a normal closing as illustrated in FIG. 6. In a specific embodiment of the damper according to the invention, the impact of the damper blades 19 against their adjacent shafts or the stops 31 is sufficient so that the flexure resulting in the damper blades 19 allows their shafts 20 to rotate in a counterclockwise direction an additional approximately beyond nonnal closed position.

As can best be seen in FIGS. 4 and 5, when the damper shafts rotate this extra 15 beyond normal closed position, the counterweights 38 are free to move downwardly relative to their respective counterweight arms 36 and to impact against the force absorbing blocks 40 which absorb the kinetic energy resulting from the abrupt movement of the counterweights 38 from their normal open positions, as illustrated in FIG. 1, to their impact positions as illustrated in FIGS. 4 and 5.

The lost motion connection provided by the slots 37 between the counterweight arms 36 and the counterweights 38, and thus between the blades 19 and the counterweights 38, reduces the mass which initially must be accelerated upon the happening of a substantial back pressure in as much as the counterweight arms 36 may move through a considerable angular movement before picking up" the mass of the counterweights 38. The lost motion connection also provides for continued movement of the counterweights 38 into energy absorbing impact against the blocks 40 after the blades 19 have stopped. This relative angular movement between the counterweights 38 and the blades 19 eliminates the excessive torquing force on the blade shafts 20 which would otherwise occur both at the initiation of the closure of the damper blades 19 in response to a sudden back pressure and upon the cessation of the rotation of the blades 19.

In a specific example of a damper embodying this modification of the invention, a structure comprising 16 damper blades 19 closing a duct approximately 60 by 45 inches, has been found to close in 0.07 seconds and to resist back pressure as high as l5 pounds per square inch gauge.

The embodiment of the invention illustrated in FIGS. 7-9, inclusive, comprises a single damper blade generally indicated by the reference number 43 which is mounted by a shaft 44. The shaft 44 extends transversely across a circular damper frame 45 which has flanges 46 and 47. The damper frame 45 is secured by its flanges 46 and 47 to a circular source duct 48 and a circular delivery duct 49, and with the source duct 48 and the delivery duct 49, forms a passageway for the delivery of gaseous fluids to an enclosure.

The blade 43 has a semi-circular rib 50 which closely embraces the shaft 44 and the blade 43 is tightly secured to the shaft 44 by a plurality of clamps 51 and rivets 52. The shaft 44 extends transversely across the damper frame 45 along a line parallel to, but spaced from the median or center line of the damper frame 45 so that the blade 43 has two opposite segments 53 and 54. The shaft 44 is joumalled in aligned bearings 55, carried by the frame 45 and one end of the shaft 44 protrudes beyond its bearing 55 outside of the frame 45. Arcuate stops 56 and 57 are welded or otherwise rigidly secured to the interior surface of the frame 45, lying in vertically spaced radial planes above and below the horizontal axis of the shaft 44, respectively.

The outwardly protruding end of the shaft 44 is slotted to receive a radially extending leaf spring arm 58 pinned or rivoted in place. The arm 58 carries a counterweight 59 on its end.

During the normal flow of gaseous fluids through the source duct 48, damper frame 45, and delivery duct 49, the larger blade segment 54 is swung downwardly with sufficient force to overcome the resultant of the gases striking the smaller segment 53 and the moment of the counterweight 59 on its arm 58. Under normal operation when flow of gases to the enclosure is terminated, the mass of the counterweight 59 swings the damper blade 43 to closed position against its upper and lower stops 56 and 57, and the counterweight moves to its normal closed position indicated by the reference number 59a in FIG. 9.

In the event of a sudden increase in pressure within the delivery duct 49, the gases forcefully swing the damper blade 43 into closed position impacting its segments 53 and 54 very sharply against the upper and lower arcuate stops 56 and 57 as shown in dotted lines in FIG. 9 and indicated by the reference number 43a.

At the initiation of a rapid closing of the damper blade 43, in response to an abrupt back pressure, the flexure of the leaf spring arm 58 allows the damper blade 43 and its shaft 44 to rotate counterclock-wise in FIGS. 7 and 9 from the fully open position to a position somewhat removed therefrom without the necessity of first overcoming the inertia of the counterweight 59.

As the blade 43 swings to closed position, the kinetic energy created by the rapid movement of the damper blade 43 is released upon impact thereof against the stops 56 and 57 and is substantially absorbed by flexure of the spring arm 58 to the position indicated by the reference number 58b. The counterweight 59 moves angularly relative to the damper blade 43 to the abnormal position shown in dotted lines in FIG. 9 and indicated by the reference number 59b.

The embodiment of the invention illustrated in FIGS. 10 and 11 is similar in its overall construction to the embodiment illustrated in FIGS. 7, 8 and 9 but is'shown as being installed in a square duct formed by a source duct 60, frame 61 and a supply duct 62. As in the embodiment illustrated in FIGS. 7-9, inclusive, a damper blade 6.3 is riveted to `a damper blade shaft 64 that is journaled by bearings 65 carried by the damper frame 61. The shaft 64 lies along a transverse line that is spaced from the median or central line of the damper frame 61 so that the damper blade 63 is divided into a larger rectangular wing 66 and a smaller rectangular wing 67. A pair of vertically spaced three-sided stops 68 and 69 are welded or otherwise secured to the inner sides of the damper frame 6l in order to stop the movement of the damper blade 63 when it is swung to closed position.

As in the case of the embodiment of the invention illustrated in FIGS. 7-9, inclusive, an end of the damper blade shaft 64 protrudes through its bearing 65 at one side of the frame 6l and is split to receive a radially extending spring arm 70 which is riveted or otherwise rigidly secured therein and the spring arm 70 carries a counterweight 71 on its free end.

In this embodiment of the invention, a shock absorbing means 72 is mounted by a bracket 73 on the exterior side of the damper frame 61 and has an impact pad 74 which is in line to be struck by the counterweight 71 but located at a position beyond the normal closed position of the counterweight 7l. The impact pad 74 is shown as being positioned on the upper end of a guide rod 75 which extends through the center of a coil spring 76 and as being guided by a three-sided pocket 75.

When the damper blade 63 is swung rapidly to closed position, the relative angular movement between the counterweight 71 and the damper blade 63 provided by the spring arm 70 allows the damper blade 63 to begin moving without the necessity of overcoming the inertia of the counterweight 71. Similarly, and again because of the relative angular movement provided between the damper blade 63 and its counterweight 71, when the damper blade 63 impacts sharply against its stops 68 and 69, the momentum of the counterweight 71 causes it to flex the spring arm 70 downwardly to a position beyond its normal closed position impacting against the impact pad 74 and delivering the kinetic energy stored therein to the shock absorbing means, in this instance, comprising the spring 76.

We claim:

l. A gaseous fluid back bination,

a. an open frame defining a part of a gas passageway leading from a source of gases to an enclosure to which such gases flow through said frame and from which enclosure a violent back draft may originate,

b. at least one damper blade shaft journalled in said frame and extending transversely across the opening in said frame,

c. a resilient damper blade fixedly mounted on said shaft along a line parallel to and spaced from the median line of said blade,

d. and counterweight means operatively connected to said shaft in a position gravitationally opposite to said blade, l. said counterweight means having a moment sufficient to swing said blade to closed position when no gases are flowing through said frame, 2. said counterweight means comprising an arm fixed to said shaft and extending radially therefrom and 3. a weight mounted at the end of said arm and movable angularly relative to said shaft.

2. A damper according to claim 1 in which the counterweight arm is a leaf spring rigidly mounted on and extending radially from the damper blade shaft and the weight is fixed on the free end of said arm.

3. A damper according to claim l having a single damper blade of plan configuration matching the cross section of the frame that is mounted on the damper blade shaft along a line spaced from the median line of said blade.

4. A damper according to claim 1 and an energy absorbing means in the path of movement of said weight and located beyond the position of said weight when said blade is in normal closed position.

pressure damper comprising in com- 5. A damper according to claim 4 in which the energy absorbing means has a resiliently displaceable impact surface and is mounted on the damper frame.

6. A gaseous fluid back pressure damper comprising, in combination,

a. an open rectangular frame defining a part of a gas passageway leading from a source of gases under desired pressure to an enclosure to which such gases flow through said frame,

b. a plurality of damper blade shafts mounted in and extending transversely across the opening in said frame in spaced parallel relationship, v

c. a resilient, rectangular damper blade mounted along one of its transversely extending edges on each of said shafts,

l. each of said blades having a length sufcient to extend across such opening and a width sufficient to extend into contact with the adjacent one of said shafts,

d. and counterweight means operatively connected to said shafts in a position gravitationally opposite to said blades, l. said counterweight means having moments sufficient to swing said blades to closed position when no gases are flowing through said frame, with the free edges of said blades contacting the down stream sides of the adjacent ones of said shafts,

2. each of said counterweight means comprising a weight and a radially extending arm fixed on the associated one of said shafts and providing for relative angular movement of said weight and said shaft, whereby a back pressure in the enclosure swingssaid blades and said counterweight means toward closed position and sharply impacts said blades against the respective ones of said shafts causing said blades to flex and to rotate said shafts and said arms beyond normal closed position, said weights thereafter moving relative to said shafts for absorbing energy.

7. A damper according to claim 6 and energy absorbing means in the path of movement of each of said weights and beyond the normal closed positions of said weights.

8. A damper according to claim 6 in which each of the counterweight arms has a lost motion connection to its respective weight.

9. A damper according to claim 8 in which the lost motion connection consists of a co-operating guide and weight mounting element, said guide extending in a direction generally tangential to the arc of movement of the damper blade and the weight mounting element is freely movable along said guide.

10. A damper according to claim 6 in which the guide is an elongated opening in the counterweight arm and the weight mounting element is a rod extending through such opening.

ll. Energy absorbing means for a gas flow damper having a plurality of parallel damper blades, each of said blades being mounted along one edge on one of a series of parallel shafts and extending into contact with the adjacent one of said shafts in normal closed position, said energy absorbing means comprising a counterweight for each of said blades,

a counterweight arm fixedly mounted on each of said shafts and extending radially therefrom generally oppositely to the associated one of said blades,

a connection between each of said counterweight shafts and the associated one of said counterweights providing for relative movement between said shaft and said weight in a direction generally tangential to the arc of movement of said arm, and

a block of tough deformable material mounted in the path of movement of each of said counterweights with its surface at a point beyond the position of said counterweight when said damper is normally closed,

whereby upon rapid closing of said damper, said blades impact against the adjacent shafts and said counterweights move into contact with said blocks for absorbing the kinetic energy of said counterweights.

12. A device according to claim 11 in which the counterweights are cylindrical in shape and the blocks have ilat contact surfaces generally tangential to the surfaces of said counterweights at points of contact. 

1. A gaseous fluid back pressure damper comprising in combination, a. an open frame defining a part of a gas passageway leading from a source of gases to an enclosure to which such gases flow through said frame and from which enclosure a violent back draft may originate, b. at least one damper blade shaft journalled in said frame and extending transversely across the opening in said frame, c. a resilient damper blade fixedly mounted on said shaft along a line parallel to and spaced from the median line of said blade, d. and counterweight means operatively connected to said shaft in a position gravitationally opposite to said blade,
 1. said counterweight means having a moment sufficient to swing said blade to closed position when no gases are flowing through said frame,
 2. said counterweight means comprising an arm fixed to said shaft and extending radially therefrom and
 3. a weight mounted at the end of said arm and movable angularly relative to said shaft.
 2. said counterweight means comprising an arm fixed to said shaft and extending radially therefrom and
 2. A damper according to claim 1 in which the counterweight arm is a leaf spring rigidly mounted on and extending radially from the damper blade shaft and the weight is fixed on the free end of said arm.
 2. each of said counterweight means comprising a weight and a radially extending arm fixed on the associated one of said shafts and providing for relative angular movement of said weight and said shaft, whereby a back pressure in the enclosure swings said blades and said counterweight means toward closed position and sharply impacts said blades against the respective ones of said shafts causing said blades to flex and to rotate said shafts and said arms beyond normal closed position, said weights thereafter moving relative to said shafts for absorbing energy.
 3. A damper according to claim 1 having a single damper blade of plan configuration matching the cross section of the frame that is mounted on the damper blade shaft along a line spaced from the median line of said blade.
 3. a weight mounted at the end of said arm and movable angularly relative to said shaft.
 4. A damper according to claim 1 and an energy absorbing means in the path of movement of said weight and located beyond the position of said weight when said blade is in normal closed position.
 5. A damper according to claim 4 in which the energy absorbing means has a resiliently displaceable impact surface and is mounted on the damper frame.
 6. A gaseous fluid back pressure damper comprising, in combination, a. an open rectangular frame defining a part of a gas passageway leading from a source of gases under desired pressure to an enclosure to which such gases flow through said frame, b. a plurality of damper blade shafts mounted in and extending transversely across the opening in said frame in spaced parallel relationship, c. a resilient, rectangular damper blade mounted along one of its transversely extending edges on each of said shafts,
 7. A damper according to claim 6 and energy absorbing means in the path of movement of each of said weights and beyond the normal closed positions of said weights.
 8. A damper according to claim 6 in which each of the counterweight arms has a lost motion connection to its respective weight.
 9. A damper according to claim 8 in which the lost motion connection consists of a co-operating guide and weight mounting element, said guide extending in a direction generally tangential to the arc of movement of the damper blade and the weight mounting element is freely movable along said guide.
 10. A damper according to claim 6 in which the guide is an elongated opening in the counterweight arm and the weight mounting element is a rod extending through such opening.
 11. Energy absorbing means for a gas flow damper having a plurality of parallel damper blades, each of said blades being mounted along one edge on one of a series of parallel shafts and extending into contact with the adjacent one of said shafts in normal closed position, said energy absorbing means comprising a counterweight for each of said blades, a counterweight arm fixedly mounted on each of said shafts and extending radially therefrom generally oppositely to the associated one of said blades, a connection between each of said counterweight shafts and the associated one of said counterweights providing for relative movement between said shaft and said weight in a direction generally tangential to the arc of movement of said arm, and a block of tough deformable material mounted in the path of movement of each of said counterweights with its surface at a point beyond the position of said counterweight when said damper is normally closed, whereby upon rapid closing of said damper, said blades impact against the adjacent shafts and said counterweights move into contact with said blocks for absorbing the kinetic energy of said counterweights.
 12. A device according to claim 11 in which the counterweights are cylindrical in shape and the blocks have flat contact surfaces generally tangential to the surfaces of said counterweights at points of contact. 